This invention relates to radiopharmaceuticals and their use in nuclear medicine. More particularly, this invention relates to a method and composition for labeling red blood cells (RBC) by orally administering a stannous-containing compound to a patient to tin the cells in vivo followed by labeling the RBC with radioactive .sup.99m Tc. This invention is particularly applicable to tests designed to image blood pools in a patient, and especially for function studies involving the heart using .sup.99m Tc-labeled red blood cells as the radiopharmaceutical.
Radiopharmaceuticals approved for general distribution and in vivo diagnosis can be used for imaging, organ flow studies, organ function studies, organ localization and dilution and excretion tests. Radiolabeling of red blood cells and their clinical and research applications in nuclear medicine constitute areas of continued interest and steady growth. Significant advances have been made so that at the present time radiolabels with sufficient in vitro and in vivo stability are available for diverse applications. .sup.99m Tc-labeled red blood cells have revolutionized the field of cardiovascular nuclear medicine by making possible the imaging of blood pools and the non-invasive evaluation of various heart parameters with minimum radiation dose and trauma to the patient. .sup.99m Tc-RBC are also used for detecting vascular malformations.
It is thought that technetium in the form of technetium pertechnetate moves in and out of the red blood cell but is not bound firmly to the cell in this chemical form. Reduced technetium, on the other hand, does not generally cross the cell membrane but does bind irreversibly with hemoglobin or other red cell components. It is also thought that binding can be achieved by reduction of the pertechnetate once within the cell, and stannous (Sn.sup.2+) compounds are the most widely used reducing agents for this purpose. In most of the current procedures, red blood cells, generally in whole blood, are contacted with stannous ions using a suitable tin (II) preparation, such as pyrophosphate, glucoheptonate, DTPA, citrate or chloride. This procedure is known as "tinning" the red blood cells and can be carried out in vitro or in vivo.
Protocols for labeling red blood cells with .sup.99m Tc by in vitro methods typically involve withdrawing a blood sample from a patient, incubating the sample with a tin-containing composition, separating the serum by centrifugation, mixing a portion of the RBC with .sup.99m Tc pertechnetate, incubating the RBC and injecting the resulting .sup.99m Tc labeled RBC into the patient. The in vitro methods are characterized by several disadvantages. They require centrifugation to separate the plasma, involve multiple transfers of red blood cells, include a number of handling steps and require multiple venous punctures with the concomitant risks of tissue and vascular damage, infection and discomfort to the patient. In vitro methods frequently require the services of a skilled technician.
In vivo methods for labeling red blood cells with .sup.99m Tc are based on the intravenous administration of a suitable amount of stannous ion to a patient followed by an in vivo incubation period, typically about 30 minutes. This is followed by the intravenous injection of .sup.99m Tc pertechnetate. While RBC labeling occurs almost immediately, the process requires two injections. Once again, multiple venous punctures increase the discomfort to the patient and raise the risk of infection and damage to the biologic systems.
Combination of the in vivo and in vitro methods has been proposed. The combined method is essentially an in vivo "tinning" procedure, wherein (Sn II) is intravenously injected into the patient, followed by withdrawal of a blood sample, incubation of the sample with .sup.99m Tc-pertechnetate to label red blood cells in vitro and reinjection of the resulting labelled RBC in plasma to the patient. While the combined in vivo-in vitro method is said to improve labelling efficiency and the subsequent imaging process, the method suffers some of the same disadvantages previously described for the in vivo method, the most serious being the requirement for multiple intravenous punctures and the complexity of the various manipulations required.
The in vitro, in vivo and combined in vitro-in vivo methods for labeling red blood cells require costly stannous chloride solutions having limited shelf-life. The existing methods also require costly and relatively complicated equipment to successfully label RBC for clinical studies.
Accordingly, there exists a need in the art for a simple, less costly and effective method and composition for radiolabelling red blood cells with .sup.99m Tc. The method should not require a centrifugation step for separating plasma from RBC. The method should be conducted in vivo to minimize technician time and eliminate the multiplicity of sample-handling steps required in the in vitro methods. The need for costly and complicated equipment should be minimized. In addition, the requirement for venous punctures should be reduced in order to minimize tissue and vascular damage in the patient and to lessen patient discomfort and the risk of infection. The method and composition should provide high uptake of .sup.99m Tc by red blood cells and be suitable for carrying out blood pool imaging studies. There should be minimal deposit of the radionuclide in background tissue that may obscure definition of a targeted blood pool organ, such as the heart. The composition should be stable and have a longer shelf-life than stannous chloride preparations for intravenous use.